Solid state lighting (SSL), e.g. LED lighting, is rapidly gaining popularity because of its energy credentials and superior lifetime compared to traditional lighting, e.g. incandescent lighting, fluorescent lighting and halogen lighting. Nevertheless, market penetration of such SSL devices is not without challenges. For example, purchase cost of SSL devices is still higher than that of equivalent traditional light sources, even though the effective cost of such SSL devices is markedly lower due to their much longer lifetime, and this realisation by consumers is one of the reasons for the increased popularity of SSL.
A more serious challenge is to produce a luminous output with an SSL device that closely resembles the luminous output distribution of an equivalent traditional light source. Most SSL devices, e.g. LEDs, act as approximate point sources that produce a Lambertian luminous distribution. This makes it necessary to shape the luminous output of such SSL devices using optical elements, in order to achieve the desired luminous output distribution.
Moreover, direct visibility of the SSL elements in such devices should be avoided for reasons of glare, and for this reason many luminaires having a light exit window that is directly observable, e.g. ceiling-mounted luminaires such as troffers, deploy indirect illumination arrangements of the light exit window, in which the light emitted by the SSL elements is projected onto a light guide or a curved reflector, which deflect the incident light towards the light exit window, thereby hiding the SSL elements from direct view and reducing the glare experienced by someone directly looking at the luminaire.
However, such solutions each have their own drawbacks. For example, a light guide-based solution typically deploys outcoupling structures, e.g. arranged in a regular pattern along the light guide, to generate a relatively homogeneous luminous output with the luminaire. In order to avoid these outcoupling structures to be visible as light spots in the luminous output of the luminaire, such a luminaire typically further comprises a diffuser in the light exit window. Such an arrangement is therefore less suitable to achieve a satisfactory unified glare rating (UGR) due to the poor control over the beam angle produced by such luminaires.
A reflector-based solution works rather well if the overall size of the luminaire is relatively modest, but becomes less satisfactory, e.g. in terms of homogeneity of the luminous output, if the width of the luminaire increases, i.e. in the direction of the optical axis of the SSL elements. This is because the SSL elements need to project their luminous outputs over greater distances such that the regions of the curved reflector distal to the SSL elements become underexposed to the luminous output, causing a dimming effect in the luminous output of the luminaire in its periphery.
In another commonly deployed arrangement, the light exit window of the luminaire is directly lit by the SSL elements through respective lenses that convert the Lambertian luminous output of the SSL elements, e.g. into a batwing distribution or the like, with the light exit window including a diffuser to avoid a spotty appearance in the light exit window due to the SSL elements being individually noticeable. As will be understood from the foregoing, such an arrangement suffers from poor control over the beam shape produced with the luminaire due to the presence of the diffuser. In addition, it is not straightforward to achieve a compact design (limited thickness) of such an arrangement due to the stacking of the SSL elements, lenses and diffuser in the thickness direction.
US 2014/0104868 A1 discloses an illumination system is described including a plurality of illumination devices, each device including (i) light-emitting elements (LEEs) arranged along a corresponding first axis; (ii) an optical extractor extending along a corresponding longitudinal axis parallel to the first axis; and (iii) a light guide positioned to receive at a first end of the light guide light emitted by the LEEs and guide it to a second end of the light guide. The optical extractor is optically coupled to the light guide at the second end and is shaped to redirect the light guided by the light guide into a range of angles on either side of the light guide. The illumination devices are connected to each other to form a polygon such that the longitudinal axes of the connected illumination devices lie in a common plane. Such an illumination system is rather costly due to the large number of components it requires, e.g. the additional optical extractor, which further increases the form factor (e.g. thickness) of the illumination system.